Interface Modification with Holistically Designed Push–Pull D–π–A Organic Small Molecule Facilitates Band Alignment Engineering, Efficient Defect Passivation, and Enhanced Hydrophobicity in Mixed Cation Planar Perovskite Solar Cells

Abstract

In perovskite solar cells, interfaces play a significant role in determining the device stability and device performance. Here, we introduce a versatile donor–π–acceptor (D–π–A) based organic small molecule (AA1) containing phenothiazine (PTZ) with a long alkyl chain as the donor unit, the vinyl-substituted thiophene moiety as a π bridge, and a rhodanine-(CN)2 moiety as an acceptor unit for the first time, and it was successfully deployed to passivate the defects at the surface and grain boundaries of a dual-cation perovskite absorber. The synthesized organic small molecule was characterized thoroughly using 1H NMR, 13C NMR, FT-IR, UV–vis, CV, TGA, and HRMS studies. The FT-IR spectral analysis and X-ray photoelectron spectroscopy (XPS) analysis confirm the interaction between the organic small molecule and the perovskite absorber. Simulated electrostatic potential surface (EPS) images obtained through the density functional theory (DFT) study reveal higher electron density over the acceptor unit of AA1, which ensures effective perovskite defect passivation. The formation of high-quality perovskite film with enhanced crystallinity, improved grain size, and band energy level alignment leads to effective charge carrier transport. The dual nature (defect passivation and optimized band energy level alignment) of AA1 passivation increases the surface photovoltage (from ∼100 to ∼155 mV) and reduces the defect density and ideality factor (∼1.94 × 1014 cm–3 from ∼7.1 × 1014 cm–3 and ∼1.64 from ∼1.88, respectively). The nonradiative recombination is suppressed along with reduced hysteresis, which leads to higher open-circuit voltage (1.09 V from 1.05 V) and power conversion efficiency. This work highlights the use of a push–pull small organic molecule which ensures effective passivation of undercoordinated Pb2+ defects and improved power conversion efficiency. The superior hydrophobic nature of the molecule results in device robustness. Finally, this all-in-one molecule wrestles the three major challenges commonly seen in perovskite solar cells, i.e., interface improvement, unhindered charge carrier transport, and device stability.

Design and synthesis of multifaceted dicyanomethylene rhodanine linked thiophene: a SnOx–perovskite dual interface modifier facilitating enhanced device performance through improved Fermi level alignment, defect passivation and reduced energy loss

“…Similar PCE drops were observed in our previous reports that correlated very well with improved FF in low light intensities (0.2 sun), which justifies the thicker spiro-OMeTAD layer that is causing the FF drop under one sun test conditions. 29,37,66…”

“…32 By the Knoevenagel condensation reaction, compound 4 was reuxed with a rhodanine derivative in chloroform and acetic acid (1 : 4) to afford the nal compound AA6. 29,33 The spectral data of stepwise products including 1 H and 13 C NMR and HPLC mass spectra are provided in the ESI (Fig. S1-S9 †).…”

“…Similar PCE drops were observed in our previous reports that correlated very well with improved FF in low light intensities (0.2 sun), which justies the thicker spiro-OMeTAD layer that is causing the FF drop under one sun test conditions. 29,37,66 To further understand the V oc improvement upon AA6 passivation, the open circuit voltage loss (DV) for PSC devices was calculated using eqn (4), 67 where E g is the perovskite absorber bandgap (E g = 1.55 eV), q is the elementary charge and V oc is the PSC device open circuit voltage. For 0.1 mg mL −1 AA6…”

“…The rationale behind the design of the AA6 molecule is as follows: (a) the extended π-conjugation that can arise due to the vinyl spacer being at the centre of the molecule helps in achieving better charge transport; 27 (b) the existence of a vinyl group between adjacent thiophene rings can favour the ordered conformation along with planarity of the structure which can facilitate π–π interaction between the stacked molecular entities and results in improved thermal stability which is a crucial parameter in device fabrication; 28 (c) electron withdrawing dicyanomethylene rhodanine units conjugated to thiophene with π electron density can cause the push–pull effect 29 and this donor–acceptor (D–A) interaction, often termed intramolecular charge-transfer (ICT), is responsible for polarisation and generation of a molecular dipole; (d) the multiple heteroatoms present with lone pair electrons – for example, the sulfur atoms on the thiophene moiety – can form bonds with the undercoordinated lead atoms which can result in defect passivation; and finally (e) tuning of the surface energy of the CSL/perovskite interface by the incorporation of this molecule.…”

Printable Perovskite Diodes for Broad‐Spectrum Multienergy X‐Ray Detection

“…This approach leverages the intrinsic defect‐tolerant structure of the perovskite, which minimizes nonradiative recombination and further provides high fluorescence quantum yields of the perovskite thin films. [ 13 ] The X‐ray image of an LM2907N voltage multiplier chip (Figure 1d) is taken using such an indirect conversion approach (see Section A‐i, Supporting Information), showing the buried circuitry in the marked area (Figure 1e). To determine the spatial resolution of this imaging technique, a Pearson fit of the line spread function was carried out (Figure 1f), yielding a maximum spatial resolution of 67.5 µm.…”